Conventionally, various propositions have been made for technology for compressing and encoding image information and, for example, Japanese Laid-Open Patent Publication No. H05-41808 discloses a technology for compressing and encoding bitmap data output from a scanner in a facsimile apparatus, etc. In this technology, the bitmap data output from the scanner are converted to run-length data and stored via FIFO memory to a DRAM buffer and, based on the control from CPU, the run-length data are sequentially read from the DRAM buffer, converted to compressed/encoded data such as MH (Modified Huffman code), MR (Modified Read code), MMR (Modified Modified Read), etc., and sequentially stored into the DRAM buffer. The compressed/encoded data stored in the DRAM buffer are output to the outside via SCSI I/F.
In the encoding mode of the MH, MR, and MMR codes, etc. generated from the run-length data, when a process for one line is completed in one page of data, an EOL (End Of Line) code is added as an end-of-line code, and when a process for one page is completed, an RTC (Return To Control) code is added as a return-to-control signal indicating that transmission of one page of data is terminated to return to control, and the data process is terminated. The RTC code is constituted by six consecutive EOL codes, for example. This data processing mode is defined in ITU (International Telecommunication Union) recommendations that establish facsimile data encoding mode, and a subsequent portion of the RTC code is normally treated as noises and is not decoded.
On the other hand, various encoding/decoding technologies are proposed for confidential communication in facsimile apparatuses. For example, in the disclosure of Japanese Laid-Open Patent Publication No. H02-94836, a confidential apparatus is disposed between own-station and opposite-station communication apparatuses connected to each other via a communication line and confidential communication can be achieved with this confidential apparatus without altering the conventional communication apparatuses. The confidential apparatus is disposed between each facsimile apparatus and switching equipment, and includes two modems, a control apparatus, and a unit with a function for encrypting and decrypting signals. When confidential communication is performed: on the transmitting side, image information sent by one facsimile apparatus over the line is input through the modem; the encrypted information is sent through another modem over the line; on the receiving side, the encrypted information sent from the transmitting side over the line is input through the modem and decrypted to reproduce the image information; and the reproduced image information is sent to the other facsimile apparatus through another modem.
However, in the invention described in the Japanese Laid-Open Patent Publication No. H02-94836, the confidential apparatus must be additionally disposed between the facsimile apparatus and the switching equipment, and the confidential communication cannot be used easily because of problems of the installation cost and installation location of the confidential apparatus. Therefore, it is desired to use the confidential communication in a more simplified configuration.
As described in Japanese Laid-Open Patent Publication No. H05-41808, since a subsequent portion of the RTC code added to the end of the one-page image data is normally treated as noises, data are not added to this portion in general, and even if data are added, the data are not decoded on the receiving side and are not utilized in data communication.
Therefore, if a protocol between the facsimile apparatuses is set so as to add and transmit/receive data that should be confidential after the RTC code and if the presence of the confidential data is recognized only between a certain caller and callee establishing this protocol, it is thought that the confidential communication can be achieved easily without disposing special equipment on the facsimile apparatus as is the case of Japanese Laid-Open Patent Publication No. H02-94836. This is not achieved in the aforementioned prior arts.